This invention relates generally to ghost cancellation systems for use with television receivers and, more particularly, to acoustic surface-wave devices suitable for use with such ghost cancellation systems.
Numerous effects are known to have an influence on the integrity of a video signal between its origin at a television transmitter and its eventual application to a television receiver picture tube for reproduction of the televised image. In particular, the presence of ghosts in a video channel is a well known phenomenon resulting from multipath transmission introduced by various forms of signal reflectors. Considering "over the air" transmissions, delayed and normally attenuated versions of the main video signal commonly arrive at the receiver from indirect transmission paths established by the presence of reflecting objects within the radiation pattern of the transmitting antenna. In cable television transmission systems, misterminations and impedance discontinuities similarly cause ghost generating reflections along portions of the signal path. In either event, the ghost is manifested in the receiver by an electrical signal having substantially the same waveform as the main signal, but generally delayed in time and normally attenuated in amplitude therefrom. Thus, the net signal arriving at the receiver consists of the sum of the desired or main signal transmitted over a direct path and a reflected signal or ghost that is normally attenuated and delayed with respect to the main signal.
Furthermore, depending on the differential delay between the direct and indirect paths, the ghost will exhibit either a positive or negative polarity with respect to the main signal. Such results because the polarity of the ghost signal alternates between positive and negative directions as the relative propagation path length difference of the main and ghosts signals change by one-half carrier wavelength. Therefore, the ghost signal can represent either a "negative" or a "positive" ghost depending on the phase relationship of the direct and indirect RF carriers.
Conventionally, ghost cancellation devices employ the technique of delay and attenuation in a feedback or feedforward path to eliminate the effect of ghosts caused by multipath transmissions. Essentially, a ghost of the correct magnitude, polarity and differential delay is synthesized from the received signal so that when the "synthetic" ghost is combined with the received signal the synthetic ghost cancels the real ghost and yields a ghost-free picture.
In early devices, adjustable lengths of cable were connected for developing the "synthetic" ghost by appropriately delaying the received signal along a branched path. The delayed signal, after being attenuated and recombined with the received signal, was therefore effective for cancelling the later arriving ghost. Employing generally similar principles, more recently proposed ghost cancellation systems commonly utilize charge transfer devices such as bucket-brigade devices to effectuate the required delay in the received signal. Exemplary of such systems is the ghost cancellation system disclosed in U.S. Pat. No. 3,935,536 issued Jan. 27, 1976 to Kimura et al which discloses the use of a bucket-brigade device connected to a summing network in feedforward relationship. On the other hand, U.S. Pat. No. 3,956,585 issued May 11, 1976 to Butler et al discloses a ghost cancellor wherein a charge transfer device is connected to a summing network in feedback relationship.
It is also known to synthesize delayed replicas of the received video signal by means of surface-wave devices. Typically, surface-wave devices used for this purpose comprise an acoustic surface-wave propagating medium having an input transducer coupled thereto for launching surface-waves in response to modulated video signals. An array of output transducers is deposited on the medium and includes means for deriving outputs by selectively tapping one or more of the transducers. Therefore, to derive an output signal having a relatively long delay a remote transducer is tapped whereas a short delay is achieved by tapping a transducer positioned in relatively close proximity to the input transducer. Devices of the latter type tend to become rather cumbersome since their overall length must be sufficient to accommodate the range of delay values expected to be encountered in a real application. Moreover, conventional surface-wave devices used in ghost cancellation systems generally provide no facility for adjusting the amplitude of the synthesized ghost nor for producing synthesized ghosts having both negative and positive polarities relative to the main signal.
It is therefore an object of the present invention to devise a new and improved surface-wave device suitable for use in a television ghost cancellation system.
It is a further object of the present invention to provide a surface-wave device suitable for use with a television ghost cancellation system which does not require the substantial use of external circuits or devices for additionally modifying a synthesized or replica ghost signal.